Rolling mills have a rotating grinding plate and a plurality of rolling bodies which are pressed, for example via hydraulic cylinders, against the grinding plate. The grinding stock is passed centrally to the rotating grinding plate and is moved between the grinding plate and the rolling bodies to the grinding plate outer edge, by the centrifugal forces acting on it. There, it is blown away upward by a sifting air flow or primary air flow, and is transported to a sifter. Coarse particles are kept back in the sifter and are fed back to the grinding plate, while fine particles leave the mill or the sifter with the sifting air flow. In this case, the aim is to adjust the mill process such that a stable grinding stock layer (=grinding bed) is formed between the rotating plate and the rolling body, preventing direct contact between the rotating plate and the rolling body and ensuring that the mill runs smoothly.
FIG. 1 illustrates one mill control system for a rolling mill such as this, that is known from practice. The mill control system shown in FIG. 1 has a mill control apparatus 11 which comprises a mill load control unit 11a, a sifting air flow control unit 11b, a sifter temperature control unit 11c, a grinding pressure control unit 11d and a sifter rotation speed control unit 11e. 
The load regulator 11a measures the mill load, for example in the form of the grinding stock mass flow supplied to a mill, compares the measured mill load with a mill load nominal variable, and then if necessary adjusts the allocator rotation speed. The sifting air flow control unit 11b measures the sifting air flow supplied to a mill 1, compares the measured sifting air flow with a sifting air flow nominal variable which is determined as a function of the instantaneous mill load, and then switches the sifting air hot-air control valve 15. The sifter temperature control unit 11c measures the temperature of the air flow leaving a sifter 7, compares the measured temperature with a temperature nominal variable which is determined as a function of the instantaneous mill load, and then switches the sifting air cold-air control valve 17. The grinding pressure control unit 11d measures the grinding pressure in the mill 1, compares the measured pressure with a grinding pressure nominal variable which is determined as a function of the instantaneous mill load, and then if necessary varies the contact pressure of the rolling bodies. Furthermore, it is possible to include the air pressure difference across the mill as a correction variable in the grinding pressure control process. The air pressure difference is the pressure difference between the mill inlet and mill outlet of the hot drying air flowing through the mill, including the carried-away fine dried coal dust particles and the water vaporized from the coal (exhaust vapors). If the pressure difference is high, then the amount of hot air contains a large amount of dust. If the difference is low, then there is less dust in the air. The extent of comminution, that is to say the amount of dust, can be influenced by means of the grinding pressure. Furthermore, the air pressure difference is also dependent on the mill load. The sifter rotation speed control unit 11e measures the rotation speed of the sifter 7, compares the measured rotation speed with a sifter rotation speed nominal variable which is determined as a function of the instantaneous mill load, and then if necessary varies the sifter rotation speed.
In certain circumstances, the mills have a tendency to “rumble”, that is to say to run very roughly with vibration. This can be caused by different or changing operating conditions, for example caused by a change to a grinding stock with different grinding characteristics and/or wear of the grinding tools (rotating plate and rolling bodies) and/or by a change in the grinding stock quality. A change in the grinding stock or a change in the grinding stock quality can result in a change in the grinding bed thickness, which in some circumstances leads to mill rumbling.
DE 44 44 794 A1 discloses a mill control method in which the vibration level of the mill is recorded continuously by means of a vibration sensor, with the recorded values being used to form a long-term mean value and a short-term mean value, by means of which a first fuzzy logic function calculates a stability degree, and in which case a second fuzzy logic function calculates the nominal value of a control variable based on the calculated stability degree, in order to achieve a desired stability degree and to control the mill for optimum operation. A method according to DE 44 44 794 A1 therefore allows only control actions which can be derived from a long-term mean value.